Gaining

I've experimentally gained weight throughout my life by overeating for a few months at a time. I'd gain about 10 pounds, but then after I stop consciously overeating, my weight drops back down to normal.

After trying heavy cream, I've gained about 30 pounds and these pounds are finally sticking even after I stop drinking it.

Is weight gained from heavy cream harder to lose? Or did I just finally break my metabolism?

You probably broke your metabolism.

I can't see how the fat cell can tell where the calories came from once it is formed.

Also, if you gained a minimal amount before, you may not have actually created new fat cells but rather increased the size of those you have. If you keep gaining you are likely to start creating new fat cells and the fat cells are forever.

I always used to yo-yo about 15 pounds, but then when I gained 30 it stuck. That was without any heavy cream, so I think it’s just a matter of the size of the gain.

Heavy cream always cause me to gain rapidly and continue to gain after I stopped using it.
How it affects you may be totally different.